MIPS: Tracing: Fix argument passing of the 32bit support with gcc 4.5

As documented in the GCC 4.5 docs [1] -mmcount-ra-address uses register
$12 to pass the stack offset of the return address to the _mcount function.
On 64-bit kernels $12 is t0 but in 32-bit kernels it is t4 so we need to
use $12 instead of t0 here to be correct for both kernel types.

[1] GCC documentation: MIPS Options
http://gcc.gnu.org/onlinedocs/gcc/MIPS-Options.html

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips <linux-mips@linux-mips.org>
Cc: David Daney <david.s.daney@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/1227/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/mcount.S

@@ -57,6 +57,12 @@
 	 move ra, AT
 	.endm
 
+/*
+ * The -mmcount-ra-address option of gcc 4.5 uses register $12 to pass
+ * the location of the parent's return address.
+ */
+#define MCOUNT_RA_ADDRESS_REG	$12
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 NESTED(ftrace_caller, PT_SIZE, ra)
@@ -70,7 +76,7 @@ _mcount:
 
 	MCOUNT_SAVE_REGS
 #ifdef KBUILD_MCOUNT_RA_ADDRESS
-	PTR_S	t0, PT_R12(sp)	/* save location of parent's return address */
+	PTR_S	MCOUNT_RA_ADDRESS_REG, PT_R12(sp)
 #endif
 
 	move	a0, ra		/* arg1: self return address */
@@ -142,9 +148,9 @@ NESTED(ftrace_graph_caller, PT_SIZE, ra)
 #ifdef CONFIG_DYNAMIC_FTRACE
 	PTR_L	a0, PT_R12(sp)
 #else
-	move	a0, t0
+	move	a0, MCOUNT_RA_ADDRESS_REG
 #endif
-	bnez	a0, 1f		/* non-leaf func: stored in t0 */
+	bnez	a0, 1f	/* non-leaf func: stored in MCOUNT_RA_ADDRESS_REG */
 	 nop
 #endif
 	PTR_LA	a0, PT_R1(sp)	/* leaf func: the location in current stack */